With the problem of global greenhouse gas as a background, there has been need for development in technology of reducing fuel consumption in an automobile. For example, there has been developed a technology of reducing fuel consumption of a vehicle by recovering kinetic energy generated during deceleration as electric power to reduce the amount of electric power to be generated during a period other than deceleration, or by converting, by means of an electric motor, electric power energy recovered in a low engine efficiency region into a driving force to assist an engine. In such a vehicle, when a driver requests acceleration, in order to realize a necessary torque, there are methods such as (1) increasing the torque of the engine, (2) increasing the torque of the electric motor, and (3) changing the gear ratio. However, there are several conceivable combinations of the magnitude of the torque and the gear ratio to be output to the engine and the electric motor. Therefore, it has been difficult to output such command value to the engine and the electric motor that the fuel consumption is minimized.
When a vehicle includes an electric storage device with a large electric storage capacity, the electric motor can be driven for a long period of time with the plenty of stored energy. Therefore, the electric motor can assist the engine at an arbitrary timing. However, as the electric storage capacity is increased, the volume of the electric storage device is also increased, which decreases the living space of the vehicle and also increases the weight of the vehicle. As a result, the fuel efficiency is decreased. Further, as the electric storage capacity is increased, there is a disadvantage in that the cost is also increased. Therefore, in the case where an electric storage device with a relatively small electric storage is used and the assistance is limited to a region in which the electric motor efficiency is high, a high assistance effect can be obtained.
In the case of a general electric motor, the electric motor is operated with higher efficiency when a semiconductor element called an inverter is used for switching than when a commutator is used for switching. Further, the maximum output torque of the electric motor tends to be larger as a rotation speed thereof is lower, and the maximum output torque thereof tends to be smaller as the rotation speed thereof is higher. Moreover, when the rotation speed of the electric motor is increased, the number of times of switching is also increased. Therefore, the loss during switching is increased, which decreases the efficiency. In view of the above, it can be said that, in order to enhance the assistance effect by the electric motor, it is desired that the assistance be limited to a region in which the rotation speed is low.
General fuel consumption rate characteristics of the engine are shown in FIG. 1. In FIG. 1, the lateral axis represents the rotation speed of the engine, and the vertical axis represents an engine torque. In FIG. 1, fuel consumption per unit work amount is shown, and concentric characteristics are shown with a peak at a predetermined torque and a predetermined rotation speed. The curve A in FIG. 1 represents a best fuel efficiency drive torque curve, which is obtained by connecting a point with minimum fuel consumption rate at each rotation speed of the engine. When the engine is controlled to be operated on this best fuel efficiency drive torque curve, the fuel consumption can be reduced. In view of the above, it is understood that the following operation is desired in order to maximally reduce the fuel consumption: The engine is operated on the best fuel efficiency drive torque curve; When the driver requests a torque larger than the best fuel efficiency drive torque curve, the operation state is held without changing the rotation speed of the engine, and the difference between the best fuel efficiency drive torque and the driver request is assisted by the torque of the electric motor.
However, when the output of the engine or the output of the electric motor is increased in response to the acceleration request of the driver, the rotation speed of the engine is also increased along with the increase of those outputs. As a result, there has been a problem in that the fuel consumption is increased.
For example, in the control device disclosed in Patent Literature 1, the gear ratio is set so as to cover the deviation between the driver request torque and the best fuel efficiency drive torque by using the power generation/driving operation of the generator motor.
Further, for example, in the control device disclosed in Patent Literature 2, the priority sequence in a control of adjusting the driving force with respect to a vehicle driving force request is set in the sequence of an engine output increase, a motor output increase, and a gear speed change in a gear ratio increasing direction.